Solar energy, or converting sunlight into electrical energy, can be competitive with other forms of energy generation if it is efficient, cheap, and safe. Thin film solar cell technology combines low manufacturing cost and relatively high efficiency, but still involves toxic and hazardous materials and processes.
Generally, solar cells work by absorbing photons, which excite electrons from a valence band to a conduction band. When connected to a circuit, the electrons in the conduction band flow to drive an electrical current. Thin film solar cells use absorber layers that exhibit direct bandgaps, allowing the cells to be only a few micrometers thick. In many thin film solar cells, the photon, or light, absorber includes elements from each of Group I (copper, silver, gold), Group III (aluminum, gallium, indium), and Group VI (sulfur, selenium, tellurium). In some cases, zinc and tin are used instead of Group III elements. In one such solar cell, the I-III-VI2 semiconductor material is Copper Indium Gallium (di)Selenide (CIGS). The material is a solid solution of copper indium selenide (often abbreviated “CIS”) and copper gallium selenide, with a chemical formula of CuInxGa(1−x)Se2, where the value of x can vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). It is a tetrahedrally-bonded semiconductor, with the chalcopyrite crystal structure, and a bandgap varying continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).
Sulfur containing CIGS thin films, also known as CIGSS, has been shown to improve solar cell efficiencies. CIGSS semiconductor materials have direct bandgap wider than that of CIGS. The bandgap is between 1.0 and about 2.5 eV, which better correspond to the solar radiation range than CIGS thin films and allows optimum solar radiation absorption. High conversion efficiencies of up to 19.5% have been reported on small-area cells. In one instance, CIGSS surface is sulfurized by annealing in hydrogen sulfide gas (H2S). Because hydrogen sulfide gas is highly toxic and therefore presents manufacturing issues, alternative sulfurization techniques involve vaporizing sulfur and sulfur compounds (e.g., indium sulfide (In2S3)) in the presence of a CIGS film. Vaporization can be carried out by heating mall amounts of sulfur or sulfur compounds in a cruicible furnace at very high temperatures next to a CIGS film. While safer, the vaporization processes that do not employ H2S are still not suitable for cost-effective mass manufacturing and improved methods are desired.